Quantum nuclear ab initio molecular dynamics study of water wires

The structure of linear water wires with an excess proton was studied at ro om temperature using ab initio path integral molecular dynamics. The ab ini tio Car-Parrinello (CP) methodology employed the density functional theory (DFT) description of the electronic structure, and the Feynman path integra l approach allowed for quantization of the nuclear degrees of freedom. Thus , the influence of proton tunneling and zero point nuclear vibrations were automatically included. Four or five water molecules were linearly arranged , with an excess proton (H*), to form tetramer and pentamer complexes, resp ectively. In classical studies of the tetramer complex, the excess proton H *, centered within the wire, formed H3O+ and H5O2+ ions with the two inner water molecules. In the pentamer complex, the H* was found attached to the inner water molecule, forming a stable H3O+ ion with two covalent, hyperext ended bonds that were hydrogen bended to neighboring water molecules on bot h opposite sides. Although the addition of nuclear quantization via path in tegrals broadened the calculated distribution functions for both complexes, the overall features were unaltered, which suggests that nuclear quantum e ffects are minimal in these small, linear clusters. However, instantaneous path integral configurations revealed the formation of an extended H7O3+ co mplex predominantly in the pentamer wire, where the excess proton H* was de localized over three adjacent water molecules simultaneously. Since the com putational demands of CP make long simulations cost-prohibitive, angular di stribution functions, requiring much longer simulation times, were obtained using an MP2-based empirical valence bond (EVB) model [Sagella, D. E.; Tuc kerman, M. E. J. Chern. Phys. 1998, 108, 2073]. Additional classical CP cal culations, where the water wire ends were solvated with additional capping waters, were also performed. In these studies, the proton was observed to b e much more mobile; proton transfer occurred along the full water wire and occasionally into the water solvation caps.